Different jetting between the front and middle cylinder.

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Luciano
Posts: 156
Joined: Sat May 10, 2003 1:01 am
Location: Sweden, Malmö

Different jetting between the front and middle cylinder.

Post by Luciano »

Hi All
I have noticed that the middle cylinder are going leaner then the two front ones...
They have NOT the same working environment. Even Honda specify that the middle cylinder should have 10mm hg less then the other two when you sync them. There is no ATAC and the expansion chamber is different also.

The middle cylinder sized earlier this year. Now it is replaced and it is still going leaner then the two front ones despite I have put larger main jet 132 and still kept 130 on the two lower ones.

The good thing is that i have noticed that the dip in power between 6000 and 7000 rpm is more or less gone when I put bigger main jet (from 130 to 132) so that the middle cylinder is not so lean :)

The question is if it is "OK" to put different jet size on the middle cylinder compared too the two front ones???? Like I said earlier is that the middle cylinder has different working environment comparing two the other two ones.
Is someone else jetting with different main jets for the middle cylinder?
Best regards, Luciano.
Torque over the whole register.

Alastair
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Re: Different jetting between the front and middle cylinder.

Post by Alastair »

Are you sure you don't have crank seal problems?

Luciano
Posts: 156
Joined: Sat May 10, 2003 1:01 am
Location: Sweden, Malmö

Re: Different jetting between the front and middle cylinder.

Post by Luciano »

Alastair wrote:Are you sure you don't have crank seal problems?

Can I have it on the cylinder no 2? I do not think it is a problem even if I had then it should suck already nice fuel mixture from any of the two other cylinders... But if I suck pure air from the base gasket then I understand why I get a lean fuel mixture or a leak in the carburettor rubber or something like that. A leak down test should be feasible.

what I am talking about is when you go to a leaner and leaner state and have the same same MJ on all of your 3 carburettors then will it bee most lightly that you will get 33% chance to size any of the 3 cylinders OR you will more likely size the cylinder no2 first before you size no1 or no3?

What do you think?
Best regards, Luciano.
Torque over the whole register.

bloodnut
Posts: 334
Joined: Sat Jul 03, 2004 1:58 am
Location: New Zealand

Re: Different jetting between the front and middle cylinder.

Post by bloodnut »

In theory you should run a leaner main jet in the rear cylinder because it is less efficient (because of the exhaust layout), most people think you should run the back cylinder richer to help cool it.

No problem with running different main jets in each carb, alot of bikes come standard that way.
if all else fails - gas it

Luciano
Posts: 156
Joined: Sat May 10, 2003 1:01 am
Location: Sweden, Malmö

Re: Different jetting between the front and middle cylinder.

Post by Luciano »

Ok if i understand it correctly then we are saying that there is nothing against to jet each cylinder as a separate unit and see them as individuals. Especially when the top no2 cylinder has different working conditions compared to no 1 and no 3.

So it is down to make some Wide Open Throttle (WOT) runns and read spark plugs for EACH cylinder and select Main Jets (MJ) accordingly.
Best regards, Luciano
Torque over the whole register.

bloodnut
Posts: 334
Joined: Sat Jul 03, 2004 1:58 am
Location: New Zealand

Re: Different jetting between the front and middle cylinder.

Post by bloodnut »

you got it.

Keep us posted on your progress.
if all else fails - gas it

Channel Hopper
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Joined: Thu Sep 23, 2004 5:26 pm

Re: Different jetting between the front and middle cylinder.

Post by Channel Hopper »

bloodnut wrote:you got it.

Keep us posted on your progress.
If I can add something (with caveat)

Assuming everything else is the same

1) similar pressure on each cylinder (on a leak test)
2) the oil pump is providing the same amount on all cylinders
3) the carburettor slides are all rising by the same amount
4) the needles are all at the same heights
5) no air leaks (anywhere - in or out)
5a) No exhaust/baffle coke up
6) pulse coil/ignition pulse to the front cylinders is not on its way out
7) there has been no excessive running at full throttle once the engine has reached optimum temperature

The only difference in the power from each cylinder will be from the ATACs doing their stuff.

In real time the rear cylinder is doing the work to continue pushing round the engine, before the other two have a go at giving a bigger thump in unison. The carburettors (if synchronised) are not dealing with this requirement unless they are also offering a lead-in to the firing/performance cycle. By lowering the slide a notch on the middle carb, the rear piston is given a slightly easier time overall, with less possibility of overheating/seizure.

A lean running rear cylinder can be rectified by altering slide height with a set of feeler gauges and without stripping the carbs from the engine, though you will need to take the airbox off and have the dexterity of a gibbon. The fuel/air ratio remains the same as the others but it is now cylinder related.


Far more NS400 engines have been wrecked as a result of jetting experiments/errors, or some simple failure to retighten the clamps around the manifold/airbox once everything else has been shown to be running true.

Luciano
Posts: 156
Joined: Sat May 10, 2003 1:01 am
Location: Sweden, Malmö

Re: Different jetting between the front and middle cylinder.

Post by Luciano »

Yes Channel Hopper you have right. There is not any difference between the front cylinders and the no2. Except the carburetor slide are rising by 10 mm Hg less then no1 and no3. And on the exhaust side is different. (But when the gas is reaching to the exhaust then the jetting is what it is...?)

The experience i have is that somewhat (and I do not know why) it looks like no2 is running leaner compared to no1 and no3.

I have modded air box, Jollymoto, aftermarket ignition (checked with strobe and no 2 and no1/3 are firing there they should). The no 2 sized for me when i was doing the first WOT to check the jetting... I did not see any fault when i changed the no2 cylinder. The no2 cylinder was coted with oil underneth but on the top there was no trace of oil i presume it was burnt away completely when the combustion was to lean. But no1 and no3 there was oil "everywhere" (i changed piston rings on all of the cylinders). And all three of them had the same jet size (MJ130).

To prevent it from happening again i mounted MJ132 only on the no2 because i saw no reason to richen the fuel mixture for no1 and no3 when i had seen the lubed internals in the piston ring replacement. (And i balanced the carburetors.) Now i see on the end of the tip on the silencer (jolly silencer are telling a lot more that the standard pipes are) that no1and no3 is still going "richer" then the no2...
Well i have to look into this once again ;)
Best regards, Luciano.
Torque over the whole register.

Channel Hopper
Posts: 1286
Joined: Thu Sep 23, 2004 5:26 pm

Re: Different jetting between the front and middle cylinder.

Post by Channel Hopper »

Carb balancing is done at idle speed, and relates to intake pressure at some 1000 rpm or under.

Lowering the slide on the middle cylinder will give an unbalanced reading on the gauges, but everything else through the rpm range will have a similar rise in overall fuel/air sucked into the cylinder.

The other option is opening the bores of the two outer carbs. There is at least 1.5mm available without changing anything else which makes small jetting increases on the two front cylinders possible. No need to touch the one feeding the rear cylinder at all and the carb bank can be set up so all slides move together.
Once again the reason is to ensure no.2 cylinder gets a slightly easier life, but you are taking the carb bank/inlets apart with the associated risks of leaving something amiss.

Luciano
Posts: 156
Joined: Sat May 10, 2003 1:01 am
Location: Sweden, Malmö

Re: Different jetting between the front and middle cylinder.

Post by Luciano »

I do not know why someone should debility make cylinder no2 "a slightly easier time overall" then it has already with the factory spec. (10 mm HG less intake pressure at idle is a lower carb slide setting.)
By lowering the slide a notch on the middle carb, the rear piston is given a slightly easier time overall, with less possibility of overheating/seizure.
There is no problem with overheating/seizure on no 2 what i know of. Or is it?

IF there is generally more seizures on no2 then on no1 and no3? I recon it is due to that somehow (i do not know why) no2 is running leaner compered to the other two. That is my findings and conclusion.

My seizure on no2 was due to a lean air fuel ratio on that cylinder.
Best regards, Luciano
Torque over the whole register.

Racer
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Re: Different jetting between the front and middle cylinder.

Post by Racer »

Luciano

I too experienced a #2 piston siezure on my NS racebike last summer. this engine is given a full tune by me. It happened after I changed from MC21 carbs, V-Force reeds and Wiseco Prolites to 29 mm machined out std carbs, std reeds and std pistons. Everything else was the same, Jollymotos, porting, squish, endcans, ignition, no filter, no top lid etc. I wanted to try this set up just to check how much HP i lossed with this carb/reed set up. RWHP layed around 70 after this change.

The MJ was 165 and SJ was 40. It seems that it could be a lean siezure on #2, both 1 and 2 was ok. It could also be a small leak from the cooling system into the #2 cylinder, spoiling the oilfilm and cause temperature to rise. After inspection of the piston, I saw that the metal between the ring lands have erroded away against the exhaust port. I experienced that the cooling temperature also riced prior to the siezeing. I think it was around 75 degrees Celsius when it happened.

Happily I have a lot of dynoprints in my archive, so when I have time I need to investigate the A/F readings of the dynoprints if the # 2 had leaner running.

You have to remember that the #2 carb have a sligthly larger powerjet than # 1 and 2 also.

Racer
ESW Rotax 250 tandem twin racer, VFR400R Racer, NS400CR Racer, NS400R street special/VFR400R racer hybrid, Kawasaki S2 350, Yamaha RD500LC. RGV250 VJ23. Bianchi Motor Sport P2 "Compe Cliente" minimoto

Racer
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Re: Different jetting between the front and middle cylinder.

Post by Racer »

Hi

I checked the dynoprint after the carb/reed change, and yes - no 2 was definatly in leaner condition :shock:
I guess a closer look on the readings could saved me this one.

Racer
ESW Rotax 250 tandem twin racer, VFR400R Racer, NS400CR Racer, NS400R street special/VFR400R racer hybrid, Kawasaki S2 350, Yamaha RD500LC. RGV250 VJ23. Bianchi Motor Sport P2 "Compe Cliente" minimoto

Channel Hopper
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Joined: Thu Sep 23, 2004 5:26 pm

Re: Different jetting between the front and middle cylinder.

Post by Channel Hopper »

When you stripped down the centre cylinder was there anything amiss with the carb rubbers, or incoming airbox clamps ?

Bloodnut did originally mention the middle pot is strangled (slightly) by the pipe, and has no ATAC to widen the powerband when the powerband is reached, meaning the jetting throughout the range is different to the other two. Getting the correct ratio at all speeds on a tuned engine is going to be a fuelling experts nightmare to achieve, making the original Honda settings for a reliable 30,000 mile engine rather exceptional.

I would still take the easy path and just use a slightly smaller carb to detune the cylinder by a couple of percent over the whole rev range, leaving the main tuning to the front cylinders, so they bang at full whack.
Easy to achieve, and far easier to double-check, but that's just me.

Racer
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Re: Different jetting between the front and middle cylinder.

Post by Racer »

Hi

If your question was aimed to me Channel Hopper, I had no air leaks between carb rubbers and carbs or above. Infact the carb rubbers I used was in good nick. My advice to Luciano is to go much higher in MJ on the no 2 cylinder. Raise from 130 to 132 have no meaning if it seize on 130. Make a change to 140 or 145 next time and tune down from there. What I believe is the case here, is that the no 2 cylinder std pipe is more of a MX pipe, with rather long tuned length compared to the no 1 and 2 pipes. When changing to Jollys, you shift the powerband and exhaust temperature on no 2 much higher than with std pipes. The difference in working conditions for each pot is biggest for th no 2 uprigth cylinder.

Racer
ESW Rotax 250 tandem twin racer, VFR400R Racer, NS400CR Racer, NS400R street special/VFR400R racer hybrid, Kawasaki S2 350, Yamaha RD500LC. RGV250 VJ23. Bianchi Motor Sport P2 "Compe Cliente" minimoto

Luciano
Posts: 156
Joined: Sat May 10, 2003 1:01 am
Location: Sweden, Malmö

Re: Different jetting between the front and middle cylinder.

Post by Luciano »

Hi.
Thank you for your replies!
This establish that no2 need a bigger MJ than the other two front cylinders when you tune it. And you are right Racer that going from 130 to 132 have no bigger meaning although it is in the right direction. I need to use bigger MJ on no2!
Hope this thread make others to be more observant on no2 when they tune there engine!
Best regards Luciano.
Torque over the whole register.

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